Since a secondary battery can be repeatedly charged and discharged, the secondary batteries are useful for a reduction in waste, and is widely used for portable devices which cannot be connected to an AC power supply, or a as backup power supply when the AC power supply is disconnected or stopped. In recent years, the enlargement of a use range such as an on-vehicle application, backup for a solar cell and the like, or a power smoothing application has been considered for the secondary batteries. Because of this, an improvement in performance such as a capacity, temperature characteristics, or safety has been increasingly required for the secondary batteries.
A nonaqueous electrolyte secondary battery as an example of the secondary battery is a secondary battery which is charged and discharged by, for example, the transfer of lithium ions between positive and negative electrodes. Since the nonaqueous electrolyte battery uses an organic solvent in an electrolyte solution, it can provide a larger voltage than that provided by a battery using an aqueous solution such as a nickel-cadmium secondary battery and a nickel metal hydride secondary battery. In nonaqueous electrolyte secondary batteries which are practically used now, lithium-containing cobalt composite oxides, lithium-containing nickel composite oxides and the like are used as a positive electrode active material, for example. Carbon-based materials and oxides containing titanium (that is, titanium-containing oxides) and the like are used as a negative electrode active material. As an electrolyte solution, those obtained by dissolving a lithium salt such as LiPF6 or LiBF4 in an organic solvent such as cyclic carbonate or chain carbonate are used. The positive electrode containing the positive electrode active material exemplified above has an average operating potential of about 3.4 to 3.8 V (vs. Li/Li+), and the maximum potential during charging of 4.1 to 4.3 V (vs. Li/Li+), for example. The carbon-based material as the negative electrode active material has an average operating potential of about 0.05 to 0.5 V (vs. Li/Li+). On the other hand, lithium titanate (Li4Ti5O12) which is most typical in the titanium-containing oxide has an average operating potential of 1.55 V (vs. Li/Li+). In the nonaqueous electrolyte secondary battery produced by combining the positive electrode containing the positive electrode active material shown above and the negative electrode containing lithium titanate, the battery voltage becomes 2.2 to 3.8 V, and the maximum charge voltage becomes 2.7 to 4.3 V.
Since the secondary battery using the titanium-containing oxide for the negative electrode can improve a charge-and-discharge cycle life, output performance, and safety, the secondary battery is put to practical use. However, it is required for the secondary battery used for on-vehicle and generation-related stationary applications to largely outperform the conventional portable device applications in terms of some performance. Particularly, a life of 10 years or more, large-current discharge performance of 5 C or 10 C and the like are required for the secondary batteries used in these applications.
Various titanium-containing oxides have been proposed. Spinel lithium titanate (Li4Ti5O12, abbreviated to LTO) has been most reported, and has been already used as a commercially-available product. Other examples of the titanium-containing oxide include monoclinic titanium dioxide (TiO2, abbreviated to TiO2 (B)), niobium titanium-containing composite oxide (for example, monoclinic Nb2TiO7, and orthorhombic niobium titanium-containing composite oxide). These can achieve a larger capacity than that of the spinel lithium titanate. However, these oxides disadvantageously cause deterioration in a capacity over charge-and-discharge cycles as compared with the spinel lithium titanate.